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hello and welcome to this presentation
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understanding hf propagation in this
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presentation will introduce you to the
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basic concepts of hf propagation and
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explain how hf propagation is influenced
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by solar activity
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H F stands for high frequency and is
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usually used to refer to frequencies in
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the range of three megahertz to thirty
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megahertz although in many cases the
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practical definition of HF can be
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extended down as low as one point five
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megahertz this corresponds to
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wavelengths in the range of about a
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hundred meters to about ten meters
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you'll sometimes also hear HF referred
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to albeit somewhat imprecisely as
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shortwave the primary use of HF is for
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long distance or even global
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communications broadcasters can reach
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listeners around the world using HF and
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this global reach is also useful in many
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government and military applications
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amateur radio operators around the world
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also frequently use an experiment with
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HF and as we'll see in this presentation
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it's the unique properties of hf
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propagation that enable long-range
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communications or even global
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communications
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although propagation @hf can provide
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worldwide communications H of
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propagation can also be highly variable
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compared to communications at other
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frequencies such as at VHF and higher as
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a practical matter this means that the
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greatest challenge in HF is finding the
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optimum frequency for communicating with
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an intended destination
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under the current propagation conditions
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before we go into more detail about how
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this is done let's briefly cover the
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three main hf propagation modes line of
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sight round wave and sky wave line of
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sight is fairly easy to understand
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signals propagate in a straight
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unobstructed path between the
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transmitter and the receiver
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line-of-sight is the only hf propagation
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mode which is fairly constant your
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ability to use line-of-sight to
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communicate with another station at a
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given location doesn't change much over
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periods of minutes hours days months
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years etc that said HF isn't a very good
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choice for line-of-sight communications
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and it's rarely used for this purpose
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because of the lower frequencies HF
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requires large antennas and bandwidth is
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also somewhat limited there also tends
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to be much more noise at HF compared to
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higher frequencies this can be a problem
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because the limited bandwidth at HF
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usually means communications are carried
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out over a M or single sideband which
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are much more sensitive to noise than
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wider bandwidth fm for these reasons
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most line-of-sight communications are
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carried out at VHF or higher not at HF
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if we don't have a direct line-of-sight
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to another station ground-wave is a
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possible solution ground waves sometimes
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called surface wave involve signals
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propagating along the surface of the
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earth interaction between the lower part
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of the transmitted wave front and the
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Earth's surface caused a wave to tilt
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forward allowing the signal to follow
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the curvature of the earth sometimes
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well beyond line of sight ground wave
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propagation is however highly dependent
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on two different factors the
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conductivity of the surface and the
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frequency of the transmitted signal in
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general higher surface conductivity
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gives better results in the form of
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greater distances that can be covered
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salt water has excellent conductivity
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especially compared to dry or rocky land
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so ground wave is a good choice for ship
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to ship or ship to shore communications
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with regards to frequency ground wave
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works best for lower frequencies for
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example the theoretical range of a 150
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watt transmitter at seven mega Hertz is
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35 kilometres over land and close to 250
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kilometres over the sea at 30 mega Hertz
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however our range Falls to only 13
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kilometers over land and just over a
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hundred kilometres at sea one of the
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most important propagation modes of HF
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is sky wave because it's skywave
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propagation that enables beyond line of
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sight or worldwide communications in sky
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wave layers of ionized particles in the
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upper atmosphere refract HF signals back
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towards the earth allowing
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communications over many thousands of
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kilometres the distances that can be
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covered by different frequencies are
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almost entirely a function of the state
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of these layers of ionized particles
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collectively referred to as the
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ionosphere in this presentation we'll
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explain the different layers of the
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ionosphere how the ionosphere is
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affected by the Sun and now we can both
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quantify the current state of the
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ionosphere and predict the future state
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of the ionosphere the incident angle or
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the angle at which a signal reaches the
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ionosphere also plays an important role
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in how far a sky wave signal will
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propagate the radiation angle of an
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antenna is primarily a function both of
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the type of antenna and the location of
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which the antenna is installed higher
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placement of an antenna usually lowers
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the radiation and incident angles and
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generally speaking the lower the
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incident angle the greater the distance
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that is covered by
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guy wave propagation note however that
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so-called skip zones may be created
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depending on radiation or incident angle
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in these zones H of signals can't be
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received either via sky wave or via
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ground wave propagation in order to
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understand skywave propagation we should
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start by explaining how ionization
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occurs in the Earth's atmosphere when
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ultraviolet energy or radiation for the
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Sun strikes gas atoms or molecules in
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the atmosphere this energy can cause
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electrons to become detached the result
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is a positive ion and more importantly a
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free electron the Earth's magnetic field
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keeps these free electrons roughly in
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place the level of ionization and the
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number of free electrons increases as
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the amount of sunlight striking given
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part of the atmosphere increases when
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that part of the atmosphere rotates away
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from the Sun that is at night the energy
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is removed and the ions recombine to
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form electrically neutral atoms or
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molecules note that recombination is a
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slower process than ionization
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atmospheric ionization increases rapidly
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at dawn but decreases less rapidly after
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dark as mentioned earlier the region of
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the Earth's atmosphere that undergoes
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this ionization is collectively called
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the ionosphere the level or density of
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ionization in the ionosphere is
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different at different altitudes and
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areas with ionization Peaks are often
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grouped into so-called layers or regions
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the layers that are important for hf
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propagation are the d layer from 60 to
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100 kilometers the e layer from 100 to
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125 kilometers and the F layer or layers
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from about 200 to 275 kilometers note
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that these are only rough numbers the
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thickness and altitude of ionospheric
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layers is never constant the reason for
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defining these different layers is that
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each of these layers will refract and/or
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absorb HF signals in different ways it's
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important to note that the ionosphere
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does not reflect signals but rather
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refract signals the different electron
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densities at different altitudes is what
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makes this refraction possible let's
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start with the lowest level the
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ionosphere the D layer the D layer only
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exists during daytime hours and
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disappears at night
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although the D layer is ionized by solar
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radiation
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the density of free electrons in the D
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layer is too low to effectively refract
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HF signals and therefore the D layer
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cannot be used for skywave propagation
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instead the D layer acts as an absorber
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of HF signals this absorption is higher
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for lower frequency signals than for
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higher frequency signals absorption also
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increases with increasing ionization so
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absorption is usually highest at midday
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for these reasons the properties of D
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layer absorption means that higher
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frequency HF signals work better during
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the day time whereas lower frequency
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signals work better at night after this
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layer has disappeared the next highest
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layer the e layer is the lowest layer of
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the ionosphere that can refract HF
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signals back towards the earth and is a
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lowest layer that supports skywave
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propagation compared to the other layers
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it's relatively thin usually around 10
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kilometers or so the e layer is much
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more dense that is ionized during the
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day but unlike the D layer it doesn't
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completely disappear at night aside for
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mostly short-range daytime
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communications and a few other special
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cases ealier propagation is not commonly
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found in hf note however that at VHF the
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e layer is very important and supports
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some rather exotic and less predictable
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propagation modes such as sporadic e
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that make long-distance communication
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over thousands of kilometres possible
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even at the relatively high frequencies
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of VHF the F layer is the most important
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for skywave propagation during the day
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the F layer splits into two sub layers F
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1 and F 2 which merge back into a single
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layer again at night compared to the D
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and E layers the height of the F layers
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changes considerably based on things
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such as time of day season and solar
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conditions more on this shortly the
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lower f1 layer primarily supports short
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to medium distance communications during
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daylight hours the f2 layer on the other
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hand is present more or less
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around-the-clock it has the highest
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altitude and the highest ionization of
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all the layers and therefore is
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responsible for the vast majority of
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long-distance hf communications
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the degree to which the different layers
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of the ionosphere refract and/or absorb
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radio frequency signals is largely a
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function of that signals frequency the
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general rule for HF sky wave
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communications is to always use the
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highest possible frequency that will
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reach a given station or destination
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this is called the maximum usable
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frequency or muff signals whose
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frequencies are higher than the moth
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will not be refracted by the ionosphere
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usually the muff increases with
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increasing ionization another important
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frequency threshold is something called
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the lowest usable frequency or luf when
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the signal frequency is at or below the
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luff communication becomes difficult or
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impossible due to signal loss or
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attenuation so we want to choose a
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frequency that's somewhere between the
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luff and the muff there is one very
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important difference though between muff
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and love because the luff is mostly
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determined by noise using higher
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transmit powers a better antenna etc can
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improve or lower the luff muff on the
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other hand is entirely a function of the
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ionosphere you can't improve or increase
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them off by using more power or a better
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antenna the muff simply is what it is
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and as we'll see shortly if the luff
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becomes greater than the muff
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no HF communication is possible
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one way to determine the mouth is purely
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through experimentation but there are
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also methods for estimating them off
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using something called the critical
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frequency the process for measuring the
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critical frequency is as follows pulses
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at various frequencies are transmitted
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vertically by equipment called ion
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asan's
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depending on the frequency of the pulse
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these pulses are returned by different
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layers of the ionosphere and we can use
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the return time to estimate the heights
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of the different layers once we reach a
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certain frequency the pulses are not
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returned by the ionosphere and instead
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continue on into space this is the
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critical frequency critical frequency is
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a function of both a current ionization
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level as well as a measurement location
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its measured regularly at hundreds of
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locations around the world
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mathematically speaking the maximum
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usable frequency is the critical
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frequency divided by the cosine of the
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angle of incidence if we send a signal
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straight up at 90 degrees muffin
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critical frequency are the same but as a
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practical matter the maximum usable
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frequency is usually estimated at three
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to five times the critical frequency
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critical frequency is one way of
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quantifying the state of the ionosphere
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but it is an active test we transmit
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signals and measure the return signals
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in addition to critical frequency there
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are three common passive methods that
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can be used to quantify the state of the
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ionosphere the first of these is sunspot
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number which can be used to predict the
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level of atmospheric ionization the
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second is the solar flux index which is
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an actual measurement of ionization
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there are also two geomagnetic indices
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the a index and the K index which give
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an indication of the impact of solar
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particles on the Earth's magnetic field
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taken together these quantities provide
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a good indication of the current state
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of the ionosphere and can be used to
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predict hf propagation let's take a look
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at each of these three quantities in a
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bit more detail sunspots are relatively
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cooler surface regions of the Sun
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relatively in this case means they have
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temperatures of about 3,000 Kelvin
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versus the normal 6,000 Kelvin seen
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elsewhere after they appear sunspots
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lasts between a few days in a few months
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sunspots are associated with powerful
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magnetic fields and these fields affect
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how much radiation is given off by the
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Sun the greater the number of sunspots
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the higher the level of solar activity
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in radio
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and because of this more sunspots
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generally means higher atmospheric
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ionization higher muff and better
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overall hf propagation the quantitative
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measure of sunspots is sunspot number
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which is a daily measurement of sunspots
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note however that sunspot number isn't
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simply a count of the number of sunspots
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it also takes into account additional
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factors like the size and grouping of
00:12:50
sunspots
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sunspot numbers recorded by a number of
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solar observatories around the world and
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sunspot number values range from 0 to a
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maximum recorded value of about 250 as
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mentioned a moment ago more sunspots or
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higher sunspot number almost always
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means better hf propagation it's also
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worth noting that sunspot data have been
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collected for almost 400 years giving us
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valuable information on how the number
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of sunspots changes over time and
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sunspot numbers do change over time
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in fact sunspot activity follows a
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roughly 11-year solar or sunspot cycle
00:13:25
as shown in this graph
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generally speaking sunspot numbers are
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usually around 150 at the peak of a
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cycle during which time a propagation is
00:13:34
very good on most frequencies including
00:13:35
higher frequencies at the bottom or
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trough of the sunspot cycle sunspot
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numbers close to zero meaning much
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poorer hf propagation given the period
00:13:45
of the sunspot cycle it should be clear
00:13:47
that sunspot cycle is only good for
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predicting long term hf propagation that
00:13:51
is in terms of years and over this time
00:13:54
period it is fairly reliable it's also
00:13:57
however worth noting that it's several
00:13:58
points in history for example in the
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late 1600s and the early 1800s sunspot
00:14:03
number stayed low for several decades
00:14:04
creating so-called minimums or minima
00:14:07
with very little solar activity the
00:14:10
reasons for these minima are still very
00:14:12
much mystery we can also quantify solar
00:14:16
activity by measuring the level of solar
00:14:18
noise or flux at a frequency of 20 800
00:14:21
megahertz these measurements are
00:14:23
reported as these solar flux index with
00:14:25
values given in so called solar flux
00:14:27
units measured solar flux values
00:14:29
generally fall in the range of about 50
00:14:31
during a solar cycle minimum to about
00:14:33
300 during a solar cycle maximum since
00:14:36
solar flux is a measurement not an
00:14:38
observation it
00:14:39
to be more consistent and reliable than
00:14:41
sunspot number but it also doesn't have
00:14:43
the same 400 year history of values
00:14:45
however solar flux values tend to
00:14:48
correlate quite well with sunspot
00:14:49
numbers like sunspot number higher
00:14:52
values of solar flux mean higher maximum
00:14:54
usable frequencies and better hf
00:14:56
propagation sunspot number and solar
00:15:00
flux index are valuable measures of
00:15:01
longer-term variations in solar
00:15:02
radiation the ionosphere is also
00:15:05
affected by shorter duration events
00:15:07
occurring on the Sun the most important
00:15:09
of these are solar flares which are a
00:15:11
type of eruption on the surface of the
00:15:12
Sun solar flares caused a rapid rise in
00:15:15
both x-ray and ultraviolet radiation as
00:15:17
well as the ejection of both low and
00:15:19
high energy particles solar flares are
00:15:22
essentially unpredictable but they do
00:15:24
occur more commonly during peaks in the
00:15:26
11-year sunspot cycle solar flares have
00:15:29
a significant effect on hf propagation
00:15:31
because they can lead to sudden
00:15:33
ionospheric disturbances polar cap
00:15:35
absorption as well as both geomagnetic
00:15:38
and ionospheric storms as the name
00:15:42
implies a sudden ionospheric
00:15:43
disturbances sudden it occurs about
00:15:46
eight and a half minutes after a flare
00:15:47
that is at the same time the flare
00:15:49
becomes visibly detectable on the earth
00:15:50
and is caused by the arrival of solar
00:15:52
radiation this radiation causes delay or
00:15:55
ionization and hence delay or absorption
00:15:57
to increase rapidly starting at the
00:16:00
lower frequencies and moving upwards the
00:16:02
affected frequencies are often almost
00:16:03
completely blacked out fortunately a
00:16:06
sudden ionosphere disturbance only
00:16:08
impacts the sunlit hemisphere and tends
00:16:10
to last a relatively short time
00:16:12
typically an hour or so and in some
00:16:15
cases smaller solar flares can actually
00:16:17
enhance hf propagation by increasing
00:16:19
ionization at higher frequencies without
00:16:21
a corresponding increase in delay or
00:16:23
absorption
00:16:25
the next effect of a solar flare is
00:16:27
something called polar cap absorption
00:16:29
the high-energy particles emitted by a
00:16:31
flare reach the earth several hours
00:16:32
later and the Earth's magnetic field
00:16:34
prevents them from entering except at
00:16:36
the poles when they enter the atmosphere
00:16:38
these particles can increase the layer
00:16:40
absorption in the polar regions and this
00:16:42
effect can last for several days during
00:16:44
this time HF signals traveling through
00:16:47
or near the poles will be blocked by
00:16:49
this increased attenuation but paths
00:16:51
that do not go near the poles may remain
00:16:53
relatively unaffected during this event
00:16:56
geomagnetic storms are caused by lower
00:16:59
energy particles arriving at the earth
00:17:00
this occurs twenty to forty hours after
00:17:03
a solar flare these particles can also
00:17:05
be generated during something called a
00:17:06
coronal mass ejection which can occur
00:17:09
independently of a solar flare in either
00:17:11
case these particles can cause
00:17:13
geomagnetic storms geomagnetic storms
00:17:16
produce visible aurora but they also can
00:17:18
interfere with GPS signals satellites in
00:17:20
general terrestrial power distribution
00:17:23
networks etc geomagnetic storms don't
00:17:26
directly interfere with hf propagation
00:17:27
but they can create ionospheric storms
00:17:30
ionospheric storms lower the maximum
00:17:32
usable frequency and degrade hf
00:17:34
propagation and as mentioned earlier if
00:17:37
the mufe becomes higher than the love
00:17:38
and i aspheric storm can create a
00:17:40
complete hf sky wave blackout one final
00:17:44
note it's possible to have a geomagnetic
00:17:46
storm without an ionospheric storm but
00:17:48
the converse is not true all ionospheric
00:17:51
storms start out as geomagnetic storms
00:17:55
sunspot number and solar flux index can
00:17:57
be used to quantify on a sphere
00:17:59
conditions but to quantify geomagnetic
00:18:01
conditions we use a and K indices in
00:18:04
general lower values for a and K mean a
00:18:07
more stable ionosphere although as we
00:18:09
just mentioned in some cases a
00:18:11
geomagnetic storm may not lead to an
00:18:13
ionospheric storm a and Kerr measured at
00:18:16
observatories around the planet and
00:18:18
these local values can be averaged to
00:18:20
produce planetary values one of the
00:18:22
biggest differences between these two
00:18:24
indices is that a is calculated daily
00:18:26
whereas K is measured every three hours
00:18:28
higher values of K indicate a current or
00:18:31
ongoing geomagnetic event whereas a is
00:18:34
useful in knowing how long this
00:18:35
disturbance has been occurring let's
00:18:38
summarize what we've learned global hf
00:18:41
communications are usually based on
00:18:42
skywave propagation rather than on
00:18:44
direct line of sight or ground wave
00:18:46
propagation in skywave signals are
00:18:48
refracted by the ionosphere although the
00:18:50
effect of some layers is more to absorb
00:18:52
signals than to refract them whether or
00:18:55
not a signal is refracted or absorbed by
00:18:57
the ionosphere is largely a function of
00:18:59
three things the frequency of the signal
00:19:01
the incident angle and the amount of
00:19:03
ionization in the upper atmosphere
00:19:05
generally speaking this ionization
00:19:07
increases during daylight hours while
00:19:09
the Sun is a loon
00:19:10
that side of the earth on a longer time
00:19:13
scale ionization also increases as the
00:19:16
number of sunspots increases the number
00:19:18
of sunspots following a roughly 11-year
00:19:20
solar cycle aside for these semi-regular
00:19:23
effects certain types of solar events
00:19:25
can unexpectedly or unpredictably
00:19:27
disrupt the ionosphere and hence hf
00:19:30
propagation solar flares are the most
00:19:32
common of these and flares can lead to
00:19:34
so-called sudden ionosphere disturbances
00:19:36
folder cap absorption and both
00:19:38
geomagnetic and ionospheric storms
00:19:41
coronal mass ejections are less common
00:19:43
but often a more severe source of
00:19:45
geomagnetic storms and finally we can
00:19:48
quantify the current state of the
00:19:50
ionosphere and/or make predictions about
00:19:52
age of propagation based on measurements
00:19:54
such as sunspot number solar flux index
00:19:56
and the A&K bag netic indices this
00:20:00
concludes our presentation
00:20:01
understanding hf propagation thanks for
00:20:04
watching
